How Many Heart Beats Do We Get?

It's a question that every runner is asked at some point: "Aren't you afraid you're using up all your heart beats by exercising so much?"

First of all, for the record, let's just do some simple math. Let's say you're a sedentary dude with a resting pulse of 60. Each day, your heart beats 24*60*60 = 86,400 times. Now let's say you're a real nut who takes up running, and works up until you can go for an hour every day with a pulse of 160 (which is likely an overestimate). As you get fitter, your resting pulse drops to 50. Now, in any given day, your heart beats 23*60*50 + 60*160 = 78,600 times. So in fact, by running, you're saving 7,800 beats every day!

The reason I bring this up is an interesting Danish study just published in the journal [4]Heart[4]. Essentially, they followed 2,800 middle-aged and older men for 16 years, and found that higher resting heart rate at the beginning of the study predicted a greater chance of dying before the end of the study. This is no big surprise -- cardiovascular fitness lowers resting heart rate and also raises life expectancy. But this study had a twist: in addition to asking the subjects how much exercise they got, they also tested their VO2max to get a reliable measure of cardiovascular fitness. Here's the relationship between resting pulse and VO2max:

This allowed them to refine the analysis, adjusting the results to eliminate the effects of physical fitness, physical activity, and various other risk factors. After all these confounders were eliminated, higher resting heart rate still resulted in elevated risk of mortality. Those with resting pulse above 90 were 3.06 times more likely to die during the study than those with resting pulse below 50, all else (including VO2max) being equal. Risk of mortality increased by 16% for each 10-beat rise in resting pulse. Here's how the lead researcher put it to the New York Times[5]:

“If you have two healthy people exactly the same in physical fitness, age, blood pressure and so on, the person with the highest resting heart rate is more likely to have a shorter life span.”

Why does this happen? The answer is basically "we don't know." The authors mention "basal metabolic effects[6]"; also, "high heart rate may promote the development of atherosclerosis and plaque rupture through increase in cardiac work, decreased artery compliance and increase in arterial wall stress." But this is all speculation. And we should bear in mind that these sorts of statistical "corrections" are never perfect, so the effect could still be mostly related to physical fitness. The way this data was collected was far from perfect -- the VO2max data was measured in 1970-71, and then the resting pulse data was measured in 1985-86, and then the number of deaths was checked in 2001. Nonetheless, it's interesting.

One of the papers they mention in the discussion section is a famous one from 1997[7] that compared the resting pulse and life expectancy of various mammals. Here's one of the graphs from that paper:

The horizontal axis shows the average number of heart beats in a lifetime (obtained by multiplying average resting pulse by life expectancy); the vertical axis shows life expectancy. On the surface, it looks pretty compelling! All mammals get roughly the same number of heart beats, and the author speculates that this has to do with the basic energetics of living matter -- the amount of oxygen consumed for each atom in an animal's body. I do have a bone to pick with this graph, though. It's clearly displayed in order to make it look like all the points are very close together. But the horizontal axis is logarithmic! It runs from 100 to 1 trillion, even though all the values are clustered around 1 billion -- with the exception of one.

Here's another graph from the same paper, in which the outlier is more obvious:

I think this is a pretty cool graph, and it does show that resting pulse and life expectancy follow a predictable relationship in mammals. But it also shows that humans are way, way off the curve. The paper mentions "advances in science, medicine and sociology" as potential reasons, but we'd need a life expectancy of about 20 to fit on the curve. It's been a long time since human life expectancy was that low!

To me, it seems very easy to believe that lower resting pulse rates correlate -- in very broad strokes -- with longer life expectancies. The animal data certainly backs that up. But the fact the humans are so far removed suggests to me that it's not a causal relationship, or at least not one with much power. (And I should emphasize that, while the 1997 paper suggests that we should investigate the possibility that pulse-lowering medications might extend life, the new Danish study does not. It suggests that elevated resting heart rate is an "independent risk factor" for mortality -- i.e. a warning sign -- rather than a cause.)